Dynamic strain ageing, deformation, and fracture behavior of modified 9Cr-1Mo steel

Tensile behavior of modified 9Cr-1Mo steel was studied over the temperature range from room temperature (RT) to 600 °C at three strain rates 10−5, 10−4 and 10−3 s−1. Serrated plastic flow was observed from 250 to 350 °C at strain rate of 10−4 s−1, signifying occurrence of dynamic strain ageing (DSA). Characteristic features of DSA such as plateau/peak in yield and tensile strength, minima in ductility, negative strain rate sensitivity, and peak in internal friction were also observed. Activation energy for DSA was found equivalent to that for diffusion of nitrogen, hence nitrogen was considered to cause DSA. Dislocation substructure in the region of DSA revealed dislocation debris, kinks, bowing of dislocations and high density of dislocations. Irrespective of temperature from 200 to 450 °C there was formation of dislocation cell structure; however, cell size in the region of DSA was smallest. Fractographic analysis showed rosette type fracture at RT resulting from longitudinal splitting. It was associated with decohesion of interface of carbide particles and prior austenite grain boundaries. Non uniform and shallow dimples were observed in fibrous zones in the region of DSA. Contours of dual shear lip zone were observed at elevated temperatures from 200 to 450 °C.